The invention relates to a method and system for calibrating a diffractive grating modulator. In particular it relates to systems that have the output system either in one of the first orders of diffraction or in the 0th order. The invention also relates to a method for calibrating the system either prior to the application of a data stream or during the application of a data stream.
Diffractive grating modulators are well known in the patent literature. A diffractive spatial light modulator formed in an electro-optic material was disclosed in U.S. Pat. No. 4,281,904, issued Aug. 4, 1981 to Sprague et al., entitled xe2x80x9cTIR Electro-Optic Modulator with Individually Addressed Electrodes.xe2x80x9d Liquid crystal diffractive modulators have been disclosed in U.S. Pat. No. 4,729,640, issued Mar. 8, 1988 to H. Sakata, entitled xe2x80x9cLiquid Crystal Light Modulation Devicexe2x80x9d; U.S. Pat. No. 4,822,146, issued Apr. 18, 1989 to Yamanobe et al., entitled xe2x80x9cOptical Modulation Elementxe2x80x9d; and U.S. Pat. No. 4,850,681, issued Jul. 25, 1989 to Yamanobe et al., entitled xe2x80x9cOptical Modulation Devicexe2x80x9d These modulators all operated with the light transmitting through the device.
Reflective diffraction grating modulators have also been disclosed in U.S. Pat. No. 4,011,009 issued Mar. 8, 1977 to Lama et al., entitled xe2x80x9cReflection Diffraction Grating Having a Controllable Blaze Angle,xe2x80x9d U.S. Pat. No. 5,115,344 issued May 19, 1992 to J. Jaskie, entitled xe2x80x9cTunable Diffraction Grating,xe2x80x9d and U.S. Pat. No. 5,222,071 issued Jun. 22, 1993 to Pezeshki et al., entitled xe2x80x9cDynamic Optical Grating Device.xe2x80x9d More recently, Bloom et al described an apparatus and method of fabrication for a device for optical beam modulation, known to one skilled in the art as a grating-light valve (GLV); see U.S. Pat. No. 5,311,360 issued May 10, 1994 to Bloom et al., entitled xe2x80x9cMethod and Apparatus for Modulating a Light Beam.xe2x80x9d This device was later described by Bloom et al with changes in the structure that included: 1) patterned raised areas beneath the ribbons to minimize contact area to obviate stiction between the ribbon and substrate; 2) an alternative device design in which the spacing between ribbons was decreased and alternate ribbons were actuated to produce good contrast; 3) solid supports to fix alternate ribbons; and 4) an alternative device design that produced a blazed grating by rotation of suspended surfaces. See U.S. Pat. No. 5,459,610 issued Oct. 17, 1995 to Bloom et al., entitled, xe2x80x9cDeformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate.xe2x80x9d Bloom et al. also presented a method for fabricating the device; see U.S. Pat. No. 5,677,783 issued Oct. 14, 1997 to Bloom et al., entitled, xe2x80x9cMethod of Making a Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate.xe2x80x9d
Recently, Bloom et al. have disclosed another form of the grating light valve in U.S. Pat. No. 5,841,579 issued Nov. 24, 1998 to Bloom et al. entitled xe2x80x9cFlat Diffraction Grating Light Valve.xe2x80x9d A method for making this form of the grating light valve is disclosed in U.S. Pat. No. 5,661,592 issued Aug. 26, 1997 to Bornstein et al., entitled, xe2x80x9cMethod of Making and an Apparatus for a Flat Diffraction Grating Light Valve.xe2x80x9d
The aforementioned diffractive modulators have been used in various display and printing systems. See U.S. Pat. No. 4,389,659 issued Jun. 21, 1983 to R. Sprague, entitled xe2x80x9cElectro-Optic Line Printerxe2x80x9d; U.S. Pat. No. 5,237,435 issued Aug. 17, 1993 to Kurematsu et al., entitled xe2x80x9cMulticolor Projector Employing Diffraction Grating Type Liquid Crystal Light Modulatorsxe2x80x9d; and U.S. Pat. No. 5,764,280 issued Jun. 9, 1998 to Bloom et al., entitled xe2x80x9cDisplay System Including an Image Generator and Movable Scanner for Same.xe2x80x9d
Methods for calibration of digital printing and display systems have been disclosed. A system and method for calibrating a Grating Light Valve was published by R. W. Corrigan et al. in xe2x80x9cCalibration of a Scanned Linear Grating Light Valve Projection System,xe2x80x9d SID ""99 Digest, pp. 220-223. E. E. Thompson discloses a calibration method for a printing system based on a digital micromirror device (DMD); see U.S. Pat. No. 5,842,088 issued Nov. 24, 1998 to E. Thompson, entitled xe2x80x9cMethod of Calibrating a Spatial Light Modulator Printing System.xe2x80x9d This method was utilized to detect and compensate for faulty or stuck pixels. A calibration system for a projection display that uses a correction factor approach was disclosed in U.S. Pat. No. 5,032,906 issued Jul. 16, 1991 to G. Um, entitled xe2x80x9cIntensity Calibration Method for Scene Projector.xe2x80x9d
Intensity stabilization methods utilizing diffractive grating modulators (specifically, acousto-optic modulators (AOM)), have been disclosed in U.S. Pat. No. 4,367,926 issued Jan. 11, 1983 to T. Hohki, entitled xe2x80x9cLight Beam Intensity Stabilizing Method,xe2x80x9d and U.S. Pat. No. 4,928,284 issued May 22, 1990 to D. Bums, entitled xe2x80x9cLaser Power Control System.xe2x80x9d Both of these methods utilized the same diffracted order for both sampling and output, and used the modulation depth of the AOM for intensity control. Because the same diffracted order is used, these methods reduce the power available to be applied to the print medium, thereby having the potential for degrading the performance of the printers. There is a need therefore for an improved calibration system and method that avoids this problem.
The need is met according to the present invention by providing a diffractive grating modulator system that includes: a diffractive grating modulator active to produce a plurality of output orders of diffraction; an illumination source for directing light onto the diffractive grating modulator; an output system arranged to receive one of the orders of diffraction from the diffractive grating modulator; a detector arranged to receive a different one of the orders of diffraction from the diffractive grating and to produce a signal representing the output of the diffractive grating modulator; an electronic driving system responsive to a data stream for operating the diffractive grating modulator; and a feedback system connected to the detector and the electronic driving system and responsive to the signal for calibrating the diffractive grating modulator.
It is advantageous that the unique properties of a diffractive grating modulator can be useful for calibration of systems employing these types of modulators. Specifically, the symmetry of diffracted orders generated by a binary diffraction grating makes it possible for one order to be used for output and the second for monitoring. Common systems employ a beam splitter in the path of the output beam to sample the output for calibration. This has disadvantages, including a reduction in output power, and a potential for introduction of optical aberrations, both of which are corrected in the present invention.
The invention can be applied to, for example, linear arrays of grating modulator elements used to generate images in printing and projection display applications. The system and method for calibrating can correct for spatial nonuniformities present in the illuminating light beam or in the diffractive grating modulator. Furthermore, temporal variations in the optical power provided by the illumination source can be corrected for using the present invention.